Area Forecast Discussion
Issued by NWS Omaha/Valley, NE
Issued by NWS Omaha/Valley, NE
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607 FXUS63 KOAX 211744 AFDOAX Area Forecast Discussion National Weather Service Omaha/Valley NE 1244 PM CDT Sat Sep 21 2024 .KEY MESSAGES... - Rain and thunderstorm chances increase this afternoon, particularly for southeast Nebraska and southwest Iowa. There is a 5-15% chance of severe storms Saturday afternoon or evening. - Widespread rain is expected Saturday night and persist Sunday, with the heaviest rainfall anticipated from midnight Sunday through noon Sunday (50-90%). Rainfall totals through Sunday of 1 to 2 inches are expected south of Harlan Iowa to Omaha/Council Bluffs to Fairbury Nebraska line. Isolated totals of up to 3 inches are possible across extreme southeast Nebraska into southwest Iowa. - One more warm day, with 90 degree readings along and south of I-80.Cooler conditions will arrive Sunday and persist through the following week, with highs dropping into the 60s and 70s. && .DISCUSSION... Issued at 400 AM CDT Sat Sep 21 2024 Today: Early this morning isolated showers persist across portions of the forecast area, with coverage expected to increase slightly as 25-45kt LLJ increases moisture transport into the region,with isentropic ascent and weak disturbance moving through periphery of H5 ridge axis providing kinematic forcing. There`s a a slim chance (5%) of a stronger storm developing with MUCAPE 1-1.5K J/kg and bulk shear values 35-40kt, but relative warm H7 and H5 temperatures will limit hail production, thus wind primary threat. Convection will wane in coverage during the morning and early afternoon hours as LLJ veers and forcing diminishes. But, expected uptick in convective development by 19-21z. By then dynamic and kinematic forcing increases due to interaction of upper level trough moving across central Canadian provinces and upper low moving out of southwest U.S. A 80-100kt ULJ moves along periphery of H5 ridge as several disturbances advect into central plains in advance of southwest U.S. upper low. H5 trough moving across Canada drives cold front through the forecast area, and will be located generally along a Logan IA to Hebron NE line by 18-19z. Convective temperatures met or exceeded by 19-21z, with deterministic soundings indicating MU CAPE 2-3K. Limiting factor will weaker bulk shear values in the 25-35kt range, but there`s enough to provide some storm organization. Nudged temperatures toward 75% NBM, that are closer to deterministic convective temperatures in the lower 90s across southeast Nebraska and southwest Iowa. Tonight: Severe weather threat expected to end by early evening as front moves south of the forecast area, but risk of moderate to heavy rainfall will continue. However, southwest U.S. upper trough slow to lift into the plains, thus large scale moist ascent will continue across the region through the overnight hours. Isentropic convergence along the 305-310K surfaces will allow convection to train across southeast Nebraska and southwest Iowa overnight, tapping into precipitable water values up to 2 inches. Evaporative cooling and low level cold advection behind the front will allow overnight temperatures to fall into the 50s. Sunday and Sunday night: Rain with embedded thunderstorms will continue into Sunday as upper trough moves across the region. Each successive model run places band of heavier rainfall to the south of forecast area, however, the past couple issuances of WPC QPF has been consistent in broad-brushing 1 to 2 inch along and south of I-80, which will help reduce current soil moisture deficits of 1 to 3 inches. The likelihood of any flash flooding or flooding appears low based on current rainfall forecast. Upper low and ULJ jet core 70-80kt will move across forecast area Monday evening, with better moisture transport shifting into lower Missouri Valley. As such expect precipitation coverage and intensity to wane by early evening,and effectively ending during the overnight hours. Lack of any appreciable sun, evaporative cooling and sustained low level cold advection will keep temperatures fall-like, with highs in the 60s. Cloud cover will keep temperatures for cooling too much Sunday night, though mid 40 readings will be possible over northeast Nebraska, with upper 40 and lower 50 degree readings elsewhere. Monday through Friday: More seasonable temperatures are expected through week, with high temperatures in the 70s and lows in the 40s and 50s. Currently the forecast looks dry. There`s a slim chance of showers Tuesday night as an open H5 trough digs in the central plains. There is variance on the strength of the low, with the operational GFS trying to create a closed low. It appears the outlier based on the EC/CMC/GEFS, and NBM keeping PoPs below 15%. Another slim chance occurs Thursday into Friday as EC/CMC and LREF depicting a compact closed low moving across the southern plains. LLJ and PIVA may be enough to generated isolated showers. Once again NBM keeping PoPs below 15%, but if low tracks a little further north, southern parts of forecast area could see PoP and thus shower coverage increase. && .AVIATION /18Z TAFS THROUGH 18Z SUNDAY/... Issued at 1232 PM CDT Sat Sep 21 2024 VFR conditions are expected through much of the period at all three TAF sites with the exception of a few showers and thunderstorms, mainly at KOMA and KLNK. Latest runs of CAMs indicate that a few thunderstorms could develop near KOMA and KLNK around 21z and continue through 00z before pushing southeast of the sites. A few strong wind gusts and small hail are possible with this initial development in addition to heavy downpours. Widespread showers with a few embedded non-severe thunderstorms will spread over much of southeast Nebraska and southwest Iowa after 04-06z and continue through much of Sunday. Although VFR conditions are expected for much of the time, we could see some deteriorating condition down to MVFR or even IFR in stronger storms. && .OAX WATCHES/WARNINGS/ADVISORIES... NE...None. IA...None. && $$ DISCUSSION...Fortin AVIATION...Kern